To prevent fires, and the resulting loss of life and property, the use of flame detectors or flame detection system is not only voluntarily adopted in many situations, but, is also required by the appropriate authority with jurisdiction for implementing the National Fire Protection Association's (NFPA) codes, standards, and regulations. Facilities faced with a constant threat of fire, such as petrochemical facilities and refineries, co-generation plants, aircraft hangers, silane gas storage facilities, gas turbines and power plants, gas compressor stations, munitions plants, airbag manufacturing plant, and so on, are examples of environments, which require constant flame detection.
To appreciate the significance of the fire detection system and process proposed by this patent application, an exemplary environment, where electrostatic coating or spraying operations are performed, is explained in some detail. However, it should be understood that the invention may be practiced in any environment faced with a threat of fire.
Electrostatic coating or spraying is a popular technique for large scale application of paint, as for example, in a production painting line. Electrostatic coating or spraying involves the movement of very small droplets of electrically charged "liquid" paint or particles of electrically charged "powder" paint from a electrically charged (40 to 120,000 volts) nozzle to the surface of a part to be coated. Most industrial operations use conventional air spray systems in which compressed air is supplied to a spray gun and to a paint container. At the gun, the compressed air mixes, rather violently, with the paint, causing it to break up into small droplets, which are propelled toward the surface of the part to be coated. The parts to be coated are transported through a coating zone by a mechanical conveyor, operated at ground potential.
Electrostatic coating of parts in a production paint line, while facilitating efficiency, environmental benefits, and many production advantages, presents an environment fraught with explosive fire hazards and safety concerns. For example, sparks are common from improperly grounded workpieces or faulty spray guns. In instances where the coating material is a paint having a volatile solvent, the danger of an explosive fire from sparking, or arcing, is, in fact, quite serious. Fires are also a possibility if electrical arcs occur between charged objects and a grounded conductor in the vicinity of flammable vapors.
Thus, in the present and the past, flame detectors have routinely been located at strategic positions in spray booths, to monitor any ignition that may occur, and to shut down the electrostatics, paint flow to the gun, and conveyors in order to cut-off the contributing factors leading to the fire.
A fire occurs largely because of three contributing factors: 1) fuel, such as, atomized paint spray, solvents, and paint residues; 2) ignition temperature derived from electrostatic corona discharges, sparking, and arcing from ungrounded workpieces, and so on; and 3) oxygen derived from the surrounding air. When a fuel's ignition temperature rises in the presence of oxygen, a fire occurs.
A mere electrical spark can cause the temperature of a fuel to exceed its ignition temperature. For example, in a matter of seconds, a liquid spray gun fire can result from an ungrounded workpiece producing sparks. An electrical spark can cause the paint (fuel) temperature at the point where the spark occurs to exceed its ignition temperature. The resulting spray gun fire can quickly produce radiant thermal energy, sufficient to raise the temperature of the nearby paint residue on the booth walls or floor, causing their temperature levels to exceed their ignition temperature. That leads the paint residue to burst into flames, without any direct contact with the spray gun.
Typically, a fire can self-extinguish, if the fuel supply or the factor contributing to the rise in ignition temperature is eliminated. If a fire fails to self-extinguish, flame detectors typically activate suppression agents to extinguish the fire to prevent major damage.
Flame detectors, which are an integral part of industrial operations such as the one described above, must meet standards set by the NFPA, which are becoming increasing more stringent. Thus, increased sensitivity, faster reaction times, and fewer false alarms are not only desirable, but, are becoming a requirement.
Conventional flame detectors currently available on the market have many drawbacks. For example, they can only sense radiant energy in one or more of either the ultraviolet, visible, or the near band infrared (IR) spectrum.
Moreover, such flame detectors are unreliable and fail to distinguish false alarms, such as those caused by radiant energy sources other than a fire. Disrupting the automated painting process in response to a false alarm has tremendous financial setbacks.
The unreliability of conventional flame detectors results from their simplistic approach to detecting fire. The most advanced ones available, at best, involve simple microprocessor (otherwise referred to as controller or microcomputer) controls such as those used in microwave ovens. Their sensitivity levels are calibrated only once, during manufacture. Typically, the sensitivity levels change as time passes, making such conventional flame detectors extremely unreliable.
Many of the conventional flame detectors utilize pyroelectric sensors, which sense only the change in radiant heat emitted from a fire. Such pyroelectric sensors depend upon temperature changes, and are susceptible to premature aging, and degraded sensitivity and stability, with passage of time.
Generally, they do not take into account natural temperature variations resulting from environmental temperature changes that occur, typically during the day, as a result of seasonal changes, or prevailing climatic conditions.
Conventional flame detectors also largely rely on their ability to detect unique narrow band spectral radiation from hot CO.sub.2 (carbon dioxide) fumes emitted by a fire. Very hot CO.sub.2 fumes from a fire emit a spike band of radiant energy, approximately 4.3 microns in magnitude. However, cold CO.sub.2, typically discharged by suppression agents or resulting from a leak, absorbs energy at 4.3 microns. Thus, cold CO.sub.2 can possibly absorb a hot CO.sub.2 spike emission from a fire. Consequently, such conventional flame detectors, in many cases, can easily miss detecting a fire.
Conventional dual frequency infrared (IR) flame detectors cause false alarms when cold CO.sub.2 is present between the fire source and the detector. Such conventional detectors utilize a dual frequency analog signal subtraction technique, which is misled into believing that a strong CO.sub.2 emission spike from a fire is detected, when, in fact, a negative absorption spike (caused by a discharge or leak) is detected. This subtraction technique senses the CO.sub.2 spike at 4.3 microns and subtracts a reference band spike at 3.8 microns. The false fire signal that results, fools the flame detector into declaring a false alarm.
Conventional ultraviolet sensors are sensitive to electrostatic spray gun fires and corona discharges from waterborne coatings, which can cause false alarms and needlessly shut down production in paint spray booths. Also, because arc welding produces copious amounts of intense ultraviolet energy ("UV"), "UV" flame detectors can detect this false fire "UV" energy even at far distances from the spray booth because of reflections. Moreover, conventional "UV" detectors are highly de-sensitized as a result of absorbing smoke and a solvent mist resulting from a fire. These absorbers serve to blind the "UV" detector. "UV" detectors can provide a false sense of security that they are operating at their optimum performance levels, when, in fact, they may be vulnerable to a catastrophic fire.
Moreover, "UV" detectors are blinded or degraded by the presence of paint or oil contaminants on their viewing window lens. Their sensing techniques do not take into account the effects of such types of degradation.
Thus, a sensitive, reliable, intelligent, and effective method and system for detecting fire, is desirable for automated industrial operations, with little or no interruptions caused by false alarms.